The present invention relates to dirt repellent silicone coating compositions. More particularly, the present invention relates to dirt repellent silicone coating compositions for use on silicone coated membrane structures.
The discovery that Teflon.RTM.-coated fiberglass could be utilized as a noncombustible, durable roof structure has initiated a transformation from simplistic temporary air supported structures to one with evergrowing potential. The impetus for the development of such fabric membrane structures was to provide roofing for large sports facilities. This led to other roofing uses such as for department stores, shopping malls, schools, exhibition buildings, industrial structures and the like. While the Teflon-coated fiberglass system has many desirable features such as durability and dirt resistance, it has a major deficiency in that light (solar) transmission is limited to approximately 10 to 15% due to the opaqueness of the Teflon.
One alternative to such Teflon-coated glass fabric is to utilize a layer of clear silicone rubber on the fabric. Not only does the silicone coating provide a substantially transparent coating, but also there is provided a coating which exhibits inertness to extreme temperatures (both heat and cold), ozone and ultraviolet light. However, silicone rubber suffers from the shortcoming that it is not dirt resistant, and in fact often picks up dirt upon exposure to the atmosphere. Accordingly, it is desirable to provide a transparent or translucent coating for silicone rubber which renders the silicone rubber resistant to dirt pickup.
One such dirt-resistant silicone rubber coating is provided in U.S. Pat. No. 3,639,155 to Hartlein et al. Therein Hartlein et al. disclose a silicone rubber which is resistant to dirt pickup comprising a base of silicone rubber having a cured coating on a surface of the silicone rubber exposed to the atmosphere, said coating being bonded to said silicone rubber and consisting essentially of, in the uncured state, a room temperature vulcanizable composition stable in the absence of moisture and curable upon exposure to moisture consisting essentially of an organopolysiloxane block copolymer consisting essentially of (A) 40 to 75 mol percent of diorganopolysiloxane units wherein the diorganosiloxane units are bonded through silicon-oxygen-silicon bonds forming a polydiorganosiloxane block having an average of from 15 to 350 diorganosiloxane units per block, said polydiorganosiloxane being at least 80 mol percent dimethylsiloxane units based on the total number of siloxane units in the polydiorganosiloxane and any remaining units being selected from the group consisting of phenylmethylsiloxane units and monomethylsiloxane units; (B) 15 to 50 mol percent organosiloxane units having an average formula: ##EQU1## where x has a value of from 1 to 1.3 inclusive and R is an organic group selected from the group consisting of aryl radicals, vinyl radicals, methyl radicals, ethyl radicals and propyl radicals, said organic groups being at least 50 percent aryl radicals based on the total number of organic groups in (B), said organosiloxane units comprise a block of at least three organosiloxane units and said organosiloxane units being selected from monoorganosiloxane units and diorganosiloxane units and (C) 3 to 25 mol percent of endblocking siloxane units of the formula: ##EQU2## where y has an average value from 1.8 to 2, R.sup.1 is an organic radical selected from the group consisting of alkyl radicals having from 1 to 5 carbon atoms, phenyl radicals and vinyl radicals and Y is a monovalent radical selected from the group consisting of acetoxy radicals, alkoxy radicals having from 1 to 5 carbon atoms, and radicals of the formula --O--N.dbd.X where X is selected from the group consisting of radicals of the formula R.sup.2 C.dbd. and ##STR1## in which each R.sup.2 is a radical selected from the group consisting of monovalent hydrocarbon radicals and halogenated monovalent hydrocarbon radicals and each R.sup.3 is selected from the group consisting of divalent hydrocarbon radicals and halogenated divalent hydrocarbon radicals, the mol percentages of (A), (B) and (C) being based on the total number of siloxane units in the organosiloxane block copolymer.
More recently Shimizu et al. in U.S. Pat. No. 4,358,558, discloses a method for forming a dust or stainproof film comprising the steps of dissolving (1) 100 parts by weight of an aminoxy group-containing organosilicon compound having an average of more than two organoaminoxy groups per molecule, (2) 10 to 1000 parts by weight of a mixture of (A) 100 parts by weight of benzene-soluble polyorganosiloxane consisting substantially of SiO.sub.2 units and (R.sup.1).sub.3 SiO.sub.0.5 units in a ratio of one mole of SiO.sub.2 to 0.4 to 1.0 mole of (R.sup.1).sub.3 SiO.sub.0.5 units wherein R.sup.1 is the same or different substituted or unsubstituted monovalent hydrocarbon group and wherein the ratio of reactive groups is 0.0004 to 1 reactive group per silicon atom, and (B) 10 to 1000 parts by weight of silanol-terminated polydiorganosiloxane having a viscosity of 30 to 2,000,000 cst. at 25.degree. C. in (3) a mixed solvent consisting essentially of (A) a volatile organosilicon compound represented by the molecular formula ##STR2## wherein R.sup.2 to R.sup.7 are hydrogen or the same or different alkyl groups, m is 0 or a positive number, and n is 3 or a positive number greater than 3, and having a boiling point of 70.degree. to 250.degree. C. under normal pressure, and (B) a hydrocarbon solvent wherein the quantity of component (3)(A) is more than five weight percent of the total quantity of components (1), (3)(A) and (3)(B), such that the total quantity of components (1) and (2) are 5 to 80 weight percent of the total quantity of components (1), (2) and (3); and applying the resulting composition to the surface of a silicone elastomer, followed by drying.
Olsen, U.S. Pat. No. 4,297,265, discloses a silicone rubber coating composition having reduced surface tension comprising (a) a first component comprising silicone rubber and a solvent therefor, said silicone rubber comprising between 50% and 70% by weight of said first component, and (b) a second component comprising particulate SiO.sub.2 having a particle size of less than 45 microns, said second component comprising 90% to 110% by weight of said component. As described by Olsen, the composition of the invention is useful for coating flexible substrates which are then used directly or secured to rigid substrates. Glass fiber cloth is said to be a particularly suitable substrate which is then coated with 2-30 mils of silicone rubber to provide a structural member.
U.S. Pat. No. 4,300,532 to Olsen discloses a solar collector comprising a framework and a collector panel held in a planar position by said framework and comprising a glass cloth coated with a light absorbing room temperature vulcanizable silicone rubber.
Nelson, in U.S. Pat. No. 3,284,406, discloses a composition consisting essentially of (1) a polysiloxane of the formula, ##STR3## in which R and R.sup.1 are phenyl or methyl and at least 80 mol percent of the R.sup.1 groups are methyl, said siloxane having a viscosity of from 500 to 500,000 cs. at 25.degree. C.; (2) from 5 to 50 percent by weight based on the weight of (1) and (2) of a copolymer of SiO.sub.2, Me.sub.3 SiO.sub.0.5 and Me.sub.2 ViSiO.sub.0.5 siloxane units in which copolymer there is from 1.5 to 3.5 percent by weight vinyl groups based on the weight of (2) and in which copolymer the ratio of total Me.sub.3 SiO.sub.0.5 and Me.sub.2 ViSiO.sub.0.5 to SiO.sub.2 units is from 0.6:1 to 1:1; (3) a compound compatible with (1) and (2) which is a siloxane containing from 0.1 to 1.7 percent by weight silicon-bonded hydrogen atoms, the remaining valences of the slicon atoms in (3) being satisfied by phenyl or methyl radicals, there being at least three silicon-bonded hydrogen atoms per molecule, and in ( 3) any hydrocarbon radicals attached to an SiH silicon being essentially all methyl radicals, the amount of (3) being such that there is from 0.75 mol of SiH per mol of vinyl radicals in (1) and (2) to 1.5 mol of SiH per mol of vinyl radicals in (1) and (2); and (4) a platinum catalyst.
Modic disclosed an organopolysiloxane composition having improved tear strength in U.S. Pat. No. 3,436,366 and which is assigned to the same assignee as the present invention. That composition contains four essential components: (1) a vinyl chainstopped polysiloxane having a viscosity of from about 50,000 to 750,000 cs. at 25.degree. C.; (2) an organopolysiloxane copolymer comprising trimethylsiloxane units, methylvinylsiloxane units and SiO.sub.2 units where from about 2.5 to 10 mole percent of the silicon atoms contain silicon-bonded vinyl groups and where the ratio of trimethylsiloxane units to SiO.sub.2 units is between 0.5:1 and 1:1; (3) a platinum catalyst and (4) an organohydrogenpolysiloxane crosslinking fluid, and optionally, (5) a finely divided inorganic filler. The unique feature disclosed by Modic is the use of component (2) which contains SiO.sub.2 units, trimethylsiloxane units and methylvinylsiloxane units.